1. Technical Field
The present invention relates to a communications network, in particular to a communications network having a communications station for example a telephone exchange, and at least one interface, for example a cabinet.
2Related Art
In recent years, particularly with increasing use of the internet, there has been an upsurge of interest in providing higher data rates to users. One objective of development in this area, in countries with a heavy historical investment on copper access networks, has been to make use of existing twisted-pair telephone lines. One result of this has been the Asymmetrical Digital Subscriber Line (ADSL) approach in which it was found that an existing copper pair from a telephone exchange to a telephone subscriber's premises could, using suitable modulation techniques, support significant downstream data rates, of the order of 1.5 Mbit/s. However the actual rate obtained in practice depends on the quality and length of the path from the exchange and an alternative proposal, providing higher data rates is to make use of the copper pair only from some point rather closer to the user. This is sometimes referred to as very high speed Digital Subscriber Line (VDSL), and these technologies tend to be referred to generically as “xDSL”.
FIG. 1 illustrates such an “fibre to the cabinet” arrangement. A telephone exchange 1 provides telephony service via cables 2 (perhaps containing 1000 twisted copper pairs) to street cabinets (or cross-connect points) 3, from which rather smaller twisted-pair cables 4 feed distribution points 5. Individual twisted pairs 6 feed from the distribution point to subscriber's premises 7 to feed telephone equipment 8. Broadband service is provided from the exchange 1 by a multiplexer/demultiplexer 9 which multiplexes signals, using ATM or SDH techniques, onto one or more optical fibres 10, feeding the cabinet 3, and similarly demultiplexes signals travelling in the opposite direction. Within the cabinet 3 is (for each fibre) an optical receiver 11 and transmitter 12, demultiplexer 13, multiplexer 14 and xDSL modems 15 which are then connected via filters 16 to the copper pairs of the cable 4 leading to the distribution point 5 and thence via the pairs 6 to the subscriber premises 7 where a filter 17 separates (in the case of downstream signals) and combines (for upstream traffic) conventional telephony signals on the one hand and xDSL signals for data equipment 18 on the other. A power supply 19 is also provided in the cabinet to supply power to the receivers 11, transmitters 12, demultiplexers 13, multiplexers 14 and modems 15.